The most important discovery that has been made by geologists is that the earth is ancient—billions of years old, rather than thousands. This discovery is fundamental to our understanding of our place in the time scale of nature and also provides temporal context for the operational scale of geological and evolutionary processes on earth. (For an eloquent discussion of “deep time”, watch this YouTube video by paleontologist Stephen Jay Gould). By the mid-19th century, most geologists had accepted the idea of an ancient earth. There were simply too many events recorded in the rock record to cram into a short 6,000-year-old chronology (the approximate figure given by Archbishop James Ussher in the 17th century). But how old is the earth in actual years before the present day? For a long time, geologists had no way to answer this question. The key breakthrough that allowed this absolute age date calculation to be made was the discovery that some minerals bear radioactive isotopes that decay at a constant rate. By knowing this rate and the relative amounts of “parent” and “daughter” isotopic material that remain in a sample, it is possible to determine the absolute age of the sample.

Claire Patterson announced in 1956 that radiometric age dating revealed the age of certain meteorites—and therefore probably the earth itself, which is thought to have formed at the same time—to be about 4.55 billion years old. Shortly thereafter, the search term “age of the earth” declined steadily within the literature, perhaps because this longstanding question had finally been answered. Figure 7 demonstrates the impact that radiometric dating methods had on interest in the question of the age of the earth.

Fig. 7. Results of Ngram analysis (1800-2000) of the following search terms: “radiometric dating” (blue line) and “age of the earth” (red line).

Jonathan Hendricks is an Assistant Professor in the Department of Geology at San Jose State University and is also a research associate at the Paleontological Research Institution, Ithaca, New York. Email Dr. Hendricks at [email protected]

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